Archean eon, archaean (other.-Greek. ρχαος ancient) one of the four eons (the period of geological history during which the eonoteme was formed combines several eras) of the Earth’s history, covering time from 4.0 to 2.5 billion. years ago. The term “archaean” was coined in 1872 by the American geologist James Dana. Archaean is divided into four eras (from latest to earliest):
At that time, the Earth did not yet have an oxygen atmosphere, but the first anaerobic organisms appeared. In the same period, many existing deposits of sulfur, graphite, iron and nickel are actively formed. Archaean and the subsequent Proterozoic enter the Precambrian time period.
Hydrosphere and atmosphere: climate
At the very beginning of the Archean era, there was little water on Earth, instead of a single ocean, there were only scattered shallow pools. The water temperature reached 70-90 ° C, which could only be observed if the Earth had a dense carbon dioxide atmosphere at that time. After all, of all possible gases, only CO2 could create an increased pressure of the atmosphere (for Archaean 8-10 bar).
There was very little nitrogen in the atmosphere of the early Archean (10-15% of the volume of the entire Archean atmosphere), there was practically no oxygen at all, and gases such as methane are unstable and quickly decompose under the influence of hard solar radiation (especially in the presence of hydroxyl ion, also while occurring in a humid atmosphere).
The temperature of the Archean atmosphere during the greenhouse effect reached almost 120 ° C. If, at the same pressure, the atmosphere in the Archaean consisted, for example, only of nitrogen, then surface temperatures would be even higher and reach 100 ° C, and the temperature during the greenhouse effect would exceed 140 ° C.
Approximately 3.4 billion. years ago, the amount of water on Earth increased significantly and the World Ocean arose, overlapping the crests of the mid-ocean ridges. As a result, the hydration of the basaltic oceanic crust noticeably increased, and the growth rate of the partial pressure of CO2 in the Late Archean atmosphere somewhat decreased. The most radical drop in CO2 pressure occurred only at the turn of the Archean and Proterozoic after the release of the Earth’s core and the associated sharp decrease in the Earth’s tectonic activity.
Due to this, in the Early Proterozoic, the smelting of oceanic basalts decreased just as sharply. The basalt layer of the oceanic crust became noticeably thinner than it was in the Archaean, and beneath it, for the first time, a serpentinite layer formed the main and constantly renewed reservoir of bound water on Earth.
Flora and fauna
There is no skeletal fauna in the Archean deposits, which serves as the basis for constructing the Phanerozoic stratigraphic scale; nevertheless, there are quite a lot of various traces of organic life here. These include the waste products of blue-green algae stromatolites, which are coral-like sedimentary formations (carbonate, less often silicic), and the waste products of bacteria oncolites.
The first reliable stromatolites were discovered only at the turn of 3.2 billion. years ago in Canada, Australia, Africa, the Urals and Siberia. Although there is evidence of the discovery of the remains of the first prokaryotes and stromatolites in sediments aged 3.8-3.5 billion. years in Australia and South Africa.
Also, in the siliceous rocks of the early Archean, peculiar filamentous algae were found, which are well preserved, in which details of the cellular structure of the body can be observed. At many stratigraphic levels, there are the smallest rounded bodies (up to 50 m in size) of algal origin, which were previously taken for spores. They are known under the name “acritarch”, or “spheromorphid”.
The fauna of the Archean is much poorer than the flora. Separate indications of the presence of animal remains in the Archean rocks refer to objects that, apparently, are of inorganic origin (Aticocania Walcott, Tefemar kites Dons, Eozoon Dawson, Brooksalla Bassler) or are products of stromatolite leaching (Carelozoon Metzger). Many Archean fossils are not fully deciphered (Udokania Leites) or do not have an exact reference (Xenusion querswalde Pompecki).
Thus, prokaryotes of two kingdoms were reliably found in the Archean zone: bacteria, predominantly chemosynthetic, anaerobic and photosynthetic cyanobionts, producing oxygen. It is possible that the first eukaryotes from the kingdom of fungi appeared in the Archaean, morphologically similar to yeast fungi.
The oldest bacterial biocenoses, t.e. communities of living organisms, which included only producers and destructors, were similar to mold films (t.n. bacterial mats), located at the bottom of reservoirs or in their coastal zone. Volcanic regions often served as oases of life, where hydrogen, sulfur and hydrogen sulfide, the main electron donors, came to the surface from the lithosphere.
Throughout almost the entire Archean era, living organisms were single-celled creatures highly dependent on natural factors. And only at the turn of the Archean and Proterozoic two major evolutionary events occurred: the sexual process and multicellularity appeared.
Haploid organisms (bacteria and blue-green algae) have one set of chromosomes. Each new mutation immediately manifests itself in their phenotype. If the mutation is beneficial, it is preserved by natural selection; if it is harmful, it is eliminated.
Haploid organisms continuously adapt to the environment, but they do not develop fundamentally new features and properties. The sexual process dramatically increases the possibility of adapting to environmental conditions, due to the creation of countless combinations in chromosomes. Diploidy, which arose simultaneously with the formed nucleus, makes it possible to preserve mutations and use them as a reserve of hereditary variability for further evolutionary transformations.
